fix new tests in absence of cupy

tests/test_sliced_iteration.py

@@ -16,6 +16,9 @@ from pystencils.simp import sympy_cse_on_assignment_list
 
 @pytest.mark.parametrize("target", [Target.CPU, Target.GPU])
 def test_sliced_iteration(target):
+    if target == Target.GPU:
+        pytest.importorskip("cupy")
+
     size = (4, 4)
 
     dh = create_data_handling(size, default_target=target, default_ghost_layers=0)